This investigation concerns itself with the biochemical mechanisms that control corneal hydration (deturgescence). Tissue cultures from rabbit corneal endothelia have been grown in order to compare their biochemistry with that of the fresh tissue cells. Cyclic AMP and cyclic GMP have been measured and compared in the fresh tissues and the tissue cultures of primary outgrowths and subcultured cells approaching confluency. It was shown that these cyclic nucleotides, which control a number of cell functions, were quite highly concentrated in these cells, were similarly concentrated in fresh tissues and subcultures, and were somewhat less concentrated in primary outgrowths.